Process for measuring and determining zeta-potential in a laminarly flowing medium for practical purposes

ABSTRACT

The invention relates to a measuring method for determining the zeta-potential ZP of a material suspended in a liquid medium which comprises streaming the suspension in a laminar flow, subjecting the system to the effect of D.C. voltage, and determining the electroporetic velocity from the change of a flow velocity of a selected particle.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a new process for measuring the zeta-potentialon the surface of a material suspended in a liquid medium. The measuringis carried out, at simultaneously regulated flow velocity and voltagevalues, in a visually controlled measuring pipe provided with twoelectrodes.

According to the experiences of Smoluchowski, M. [Handbuch derElektrizitat und Magnetismus, Ed. L. Gratz, (Barth Verlag, Leipzig 1921)Volume II, page 366] if a material suspended in liquid is subjected tothe effect of steady voltage, the particles migrate. Based on theelectrophoretic velocity (migration speed) (v_(e) =cm.s⁻¹) and theelectric voltage density applied (E=V.cm⁻¹, wherein V=voltage in Voltsand cm=the distance between the electrodes in cm) the zeta-potential(ZP) characteristic for the material can be determined, while thepositive or negative polarity thereof depends on the direction of themigration of the particles towards the cathode or the anode.Zeta-potential can be calculated on the basis of the following formulae:##EQU1## wherein η stands for the viscosity of the liquid medium, εrepresents dielectric permittivity and 9.10⁴ is the conversion factor ofthe electrostatic unit into Volts.

In case of water, A=150 at a temperature of 20° C. (Martin, A. N.Swarbrick, J. Cammarata, A: Physical Pharmacy) (Lea and Febiger,Philadelphia 1969, page 458). Determination of ZP on the basis ofelectrophoretic velocity is carried out in a measuring cell providedwith molybdenum or platinum electrodes placed under a special colloidalmicroscope. The migration speed of the particles can be determined understable D.C. voltage of known value after a certain time, in a stationarystate, by means of a stop-watch, and an ocular micrometer, whereafterthe ZP value can be calculated according to the aforementioned formula.

The unavoidable disadvantage of this widely used method is that in thecourse of electrophoresis, in particular in a medium which alsocomprises electrolytes, the temperature of the system continuouslychanges. Thermic flow considerably disturbs electrophoretic migration.In addition, gas evolves on the electrodes, electrochemical processesoccur, and in the measuring cell, ions migrate, which causes adeleterious effect on the accuracy of the measurement. The fact that theliquid medium and the particles migrate in the opposite direction alongthe wall of the cell due to electroendoosmosis makes the measurementespecially difficult. Along the vertical wall of the measuring cell theparticles migrate with a changing velocity having a parabolic velocityprofile. According to said velocity profile, particle velocity reachesits maximum at the axis of the measuring cell, while at the limit ofendoosmosis it equals zero. Between this limit and the wall of themeasuring cell the particles migrate in the opposite direction.

From the foregoing it is obvious that measuring of ZP on the basis ofthe electrophoretic migration speed is very difficult. Complicatedapparatus is required; in addition, measuring errors frequently exceed10%.

In the prior art several other ZP measuring methods are known. Theseinclude the method based on measuring the electroosmosis formed underthe influence of electric field of force (Biefer, G. J., Mason S. G.:Colloid Sci. 9, 20) (1954); or on measuring flow potential (among othersMartin, W. McK., Gortner, R. A.: J. Phys. Chem. 34 1509) (1930); or onthe basis of measuring volumetric flow or potential of sedimentation.According to specific embodiments of these techniques, the measurementmay be carried out in double microcapillary tubes in an electrophoreticcell; or using the phenomena of zone-electrophoresis, the relationshipof moving phase borders and mass flow; or by electrophoretic lightdispersion, laser technique (Robert J. Hunter: Zeta Potential in ColloidScience (Acad. Press London, 1981), pages 127-175). However, suchapparatus is either not commercially available or if available, it isvery expensive. Additionally, these methods of determining ZP arelaborious and time consuming. Moreover, accuracy of measurement islimited to a variation coefficient of ±5%. In several professionalfields, among others in the chemical and pharmaceutical industry, whenplant-protecting agents, cosmetics and drugs are produced, or when thefiltering processes are optimized in the chemical industry, sewagetreatment, and drinking water purification, the accurate determinationof the ZP-value of colloids and suspensions is critical.

Stabilization of suspensions and prevention of the aggregation of thesuspended particles is possible by regulation of the ZP-value and properchoice of the additives on basis of ZP-measurements. In technicalliterature it has been generally accepted that suspensions with aZP-value of -100 nV are extraordinarily stable. In order to increaseefficiency of filtering processes (the aggregation of the particles) theZP-value should be reduced. Proper choice of additives or conditionsalso requires measuring of the ZP-value.

An object of the invention is to improve the known techniques ofmeasuring of ZP-value in order to develop a process which is easy toconduct, requires less work, and provides accurate measurements.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparative graph of measurements of zeta potential vs. pHaccording to the prior art and according to the invention;

FIG. 2 is a schematic of the apparatus for carrying out the method ofthe invention.

DESCRIPTION OF THE INVENTION

Surprisingly we have found that the electrophoretic migration ofsuspended material particles can be produced not only in a D.C. voltagefield of force. But by allowing the suspension to flow laminarly andequally in time in an adequate apparatus, and by switching on theproper + or - pole in the direction of flow and applying momentarycontrolled direct voltage of proper magnitude it may be produced.According to the process of the invention, the v_(e) electrophoreticmigration speed can be calculated in another way than described in theprior art. It is observed that local flow velocity (v) of the selectedparticle changes to the flow velocity v_(V) due to the effect of thevoltage V applied. In this case the particle does not stop but travelsslower or faster depending on the energized direction of poles. In case,the negative pole is switched in the direction of flow and flow velocityof the particle decreases upon the effect of the voltage V, the sign ofthe ZP-value will be negative; otherwise it is positive. On the basis ofthe expression

    v±v.sub.V =v.sub.e

electrophoretic migration speed v_(e) can be calculated (expressed inunits cm.s⁻¹) and in such a manner ZP can be given by using the formulaas described.

According to a preferred embodiment of the process, in the constantlyflowing liquid the particle intended to be observed will stop, as themagnitude of the electrophoretic velocity arising under the effect ofthe voltage complies with the velocity of liquid flow. ZP-value can becalculated by using the previously described equation, considering thatthe velocity of flow of the particle corresponds to the value v_(e).

According to a preferred embodiment of the method of the invention, two0.2 mm thick platinum electrodes 10 spaced apart 10.0 cm are installedin a cylindrical glasspipe 11 having an inner diameter of 3 to 6 mm,preferably 4 mm. Thereafter the measuring pipe 11 is placed under amicroscope 12 suitable for 15-30-fold magnification and provided with acalibrated ocular micrometer scale. The suspension is illuminated byfibre-optics or by means of a 100 micron optical slot 13 arranged beforethe microscope lamp 14 to eliminate the disturbing (heating) effect ofthe light source. One end of the measuring pipe is coupled by asynthetic connection 15 to an injector 16 of 2-10 cm³, containing thesuspension to be tested, while the other end is coupled by a syntheticconnection 17 to a vessel 18 for receiving the suspension flowingthrough the measuring pipe. When entering the suspension into thesystem, care is to be taken to avoid air bubbles when filling themeasuring pipe 11, the connecting synthetic pipes 15, 17 and theinjector 16. Thereafter the injector 16 is fitted into the equipment andthe suspension is allowed to flow at a regulated, steady-state velocity.The electrodes 10 (made of platinum) of the measuring pipe are connectedto a D.C. voltage supply unit 19 provided with an alternator or polechanger switch 20 and with a stabilized voltmeter 21 which works in thevoltage range between 1 and 800 V. Steady flow of the particles in themicroscope 22 can be controlled by measuring the travelling time of theselected particle, observing the same through the 100-scale of theocular micrometer, with an accuracy of 0.01 s, separately in the firstand second 50--50 pitch of the scale. If the particle takes the firstand second 50 pitch within about the same time (0.01-0.02 s differenceis acceptable), the flow in the system is considered to be steady-state,laminar. Thereafter, another particle is selected and the travellingtime thereof is measured at an optional distance (10, 20, . . . 50ocular pitch) with an accuracy of 0.01 s. Then the D.C. voltage 19 isswitched on, the pole changer 20 is positioned, and by regulatingvoltage controller 22 the voltage applied to the measuring pipe 11 themotion of the chosen particle should stop. This voltage value isregistered. The flow velocity measured with the calibrated ocularmicrometer with an accuracy of 0.01 s is calculated to V_(e) =cm.s⁻ 1units with an accuracy of four digits. Dividing the voltage needed forstopping the particle by the distance of the electrodes (in the presentcase, ten cm.), the value E of voltage density is obtained. In thismanner using the known formula ##EQU2## the ZP-value of the desiredaqueous suspension can be calculated.

It is recommended to perform measurements of a large series of particlesmigrating with different speeds. Thus, the average of the ZP-values anda variation coefficient or standard deviation can be calculated by usingknown mathematic statistical methods. These values characterize theaccuracy of measuring.

The testing method may be varied, e.g. by scanning the flowingparticles, a ZP-value of the selected particle can be determined bysimultaneously regulating flow velocity and the voltage applied to themeasuring pipe.

According to a preferred embodiment of the measuring system apparatussuitable for carrying out the method of the invention, includes ameasuring pipe 11 with an inner diameter--within the range 3 to 6 mm--,into which platinum electrodes 10 (dia. 0.2 mm) at a spacing of 10.0 cmare installed. The measuring pipe is fixed with the usual clampingplates to the stage of the microscope 22. Illumination is provided byfibre-optics instead of a condenser or with a 100 micron optical slot 13arranged in front of the condenser. The ocular micrometer is calibratedand the path length of the particles is calculated in cm. For quicklysedimenting suspensions, it is advantageous to position the microscopeat an angle of 90° with the stage of the microscope turned from thehorizontal to vertical. In this case, the sedimenting particle bendsfrom the line of the ocular micrometer scale, making it possible toexamine only those particles which are travelling parallel with the axisof the measuring pipe and with the scale line of the ocular micrometer.

The sample to be tested is fed by a syringe 16 (injector). Practically,for low velocities an injector with a smaller volume is used; if highervelocities are needed, injectors with a larger volume should be used.For this purpose the so-called tuberculine injectors with a volume of 2,5, perhaps 10 cm³ may be used. Injectors with a far larger volume arerecommended only in the case of extremely high ZP-values.

The connecting pipes 15, 17 are made of polyethylene or any otherstructural material. The sample feeder 23 is advantageously theHungarian "INFUCONT" quartz-controlled electronic apparatus, which iswell suited for the control of the "INFUMAT or INFUDRIVER" drives whichdrive the piston of injector. The electric energy at the properfrequency, phase, and voltage, required for operating the shifting-motordrive at a continuously controllable rate of feeding is electronicallydigitally displayed. The digital display (D) includes the feeder andflow velocity observed in the measuring pipe (with an inner diameter of5.0 mm (expressed in cm.s⁻¹) controlled with a 10 cm³ syntheticsyringe). Within the digital display range 0-40, the apparatus deliveredthe aqueous suspension with an acceptable linearity. In the describedarrangement the linear regression coefficient

    (cm.s.sup.-1)=0.00141. D

was found to be 0.9999. The solution of the linear equation will yielddifferent values, when using measuring pipes or injectors with differentinner diameters. Thus calibration is required for individualarrangements of given dimensions.

It is well known that a pure laminar flow will be established in acylindrical pipe within the range of contemplated velocity, based on thelow Reynolds-number. The velocity distribution curve has a parabolicshape. Along the axis of the pipe, the flow velocity is double theaverage flow velocity (v). The optimal radius r of the cross-section ofthe radius r_(o) the local flow velocity v can be calculated on basis ofthe Knudsen and Katz rule ##EQU3## (John H. Perry: Manual of ChemicalEngineers, Technical Publishers, Budapest, 1953. Volume I, page 564).From this it becomes obvious that in the microscope, in dependence ofthe depth of focus, at a relatively high velocity (above 100micron.s⁻¹), parabolic velocity distribution becomes flattened to suchan extent that in its visual field particles travelling with differentspeeds can be observed. At considerably lower velocities particlestravelling with nearly identical velocities can be observed.

According to an improved mode of realization of the process according tothe invention, the measuring pipe 11 is placed on the microscope 12--asdetailed previously--, the injector 16 is arranged in the feeding device23 and connection is made with synthetic pipes 15, 17. The feeder 23 iscalibrated for the suspension to be examined, as specified earlier. Theflow velocity of the particles is twice the digital signal of thefeeder. By the aid of the voltage regulator of the D.C. voltage currentsource 19, the voltage needed for stopping the particles is adjusted.Care should be taken that the pole switch 20 is in the proper direction,in compliance with the nature of the material tested.

The feeder 23 and the D.C. voltage supply unit 19 deliver an electricsignal proportional with the flow velocity and the voltage value neededfor stopping the particles, these two signals are fed to a computer anddisplay unit. Thereafter, based on a predetermined algorithm, theZP-value of the material tested is obtained automatically.

Accuracy of the measuring system and the testing method are illustratedby the following examples

Example 1

2-3 mg of 17α-hydroxy-21-acetoxy-corticosterone, as a basic materialwith a grain size below 10 microns, are mixed with a small quantity ofwater in a frictional mortar. The mixture thus obtained is diluted with50 cm³ of distilled water and allowed to stand for three days.Measurement is obtained with a measuring pipe having an internaldiameter of 4.0 nm with electrodes arranged therein with a spacing of 10cm; a 5 cm³ synthetic injector; and a microscope with 10 Xmagnification. The flow time on the 1000 micron path in second(s), aswell as voltage required for stopping the particle are determined.

Thereafter further measuring is performed for a 318 microns path with 30X magnification. ZP-values calculated from the measured data are asfollows:

    ______________________________________                                        s               V      ZP                                                     ______________________________________                                        1000 microns path                                                             11.0            230    -59.3                                                  12.0            222    -56.3                                                  12.0            222    -56.3                                                  318 microns path                                                              3.80            220    -57.1                                                  3.73            220    -58.1                                                  3.73            219    -58.4                                                  3.93            205    -59.2                                                  3.87            208    -59.3                                                  ______________________________________                                    

Average value: -58.0 coefficient of variation ±1.2 (±2.07%).

These test results demonstrate well that the measuring system and thetesting method of the invention can be used for determining the ZP-valuewith an accuracy which corresponds to and often exceeds the experiencesreported heretofore in technical literature.

In technical literature ZP-value has been referred to generally as an"apparent" value, form which conclusions can be drawn on theinaccuracies resulting from the difficulties of determination. Takinginto consideration that measuring technique based on a flow systemeliminates numerous disturbing factors, a ZP-value determined in thatway may be considered as a virtual value. In order to be able todemonstrate reproducibility of the data of technical literature,comparative tests were also carried out.

Example 2 (comparative example)

In the earlier described manner, colloidal iron(III)hydroxide was testedin freshly boiled and cooled distilled water at 22° C. after havingprecipitated ferric-chloride with sodium hydroxide and eliminatedchlorine. Measured results:

    ______________________________________                                        318 microns path:                                                             s              V      ZP mV                                                   ______________________________________                                        4.58           235    -44.3                                                   4.80           220    -45.1                                                   4.76           222    -45.1                                                   4.90           215    -45.3                                                   5.00           231    -41.3                                                   5.57           203    -42.2                                                   6.43           181    -41.0                                                   7.12           162    -41.3                                                   ______________________________________                                    

Average ZP value: -43.23 mV

Variation coefficient: ±1.78 (±4.11%)

The value given in the technical literature is 45 mV; the difference is-3.93% (Martin, A. N. et al. Physical Pharmacy (Lea and Febiger,Philadelphia 1969) page 458).

ZP-value is considerably influenced by traces of carbonic acidimpurities in water, or chloride ion impurities of the sample; thecomparison to the value of technical literature is considered good.

Example 3 (Comparative Example)

The second coparative test performed for determining ZP-values inalumina/10⁻⁴ M potassium-nitrate solution in dependence of pH wascarried out as previously described (Wiese, R. G. et al.: ColloidInterface Sci. 51, 427) (1975)); the results obtained are reflected inFIG. 1.

FIG. 1 illustrates that the ZP-values measured in the flowing systemshow a good comparison with the data of technical literature. Differenceof larger extent, as observed in high pH-ranges may result from thedifferences in the structure of the solid material tested in addition tothe error sources as mentioned before.

Example 4 Comparative test for comparing the ZP-values of polymorphouscrystal modifications

It is well known that some elements have allotropic structure. Organiccompounds can be produce in several crystal modifications, e.g. thepesticidal pharmaceutical basic material Mebendazol, the threepolymorphous crystal modifications (A, B and C) of which are known(Lancos; Krisztina: Ph.D. thesis, Medical University Semmelweis,Pharmaceutical Institute, Budapest, 1985). In respect to chemicalcomposition, the single Mebendazol polymorphes are quite identical,however, the structure of their solid body is reversibly different.Three Mebendazol polymorphous crystal modifications we comminuted to agrain size finer than 30 microns and 2 mg thereof were suspended indistilled water. ZP-value was measured in a flowing system at 20° C.temperature, as specified earlier. The average value of six measurementsand variation coefficient of the measuring series were, as follows:

    ______________________________________                                        sample             ZP.sub.mV                                                                            ±                                                ______________________________________                                        Mebendazol A       106.3  2.1                                                 Mebendazol B        34.9  0.9                                                 Mebendazol C       104.0  3.0                                                 ______________________________________                                    

Example 5 Stabilization of the suspension by controlling the ZP-value byusing additives

According to technical literature stability of a suspension isconsidered as maximally satisfactory, if the value of ZP isapproximately -100 mV. The stability of the suspension does not meet therequirements if the ZP-value thereof is lower; under -40 agglomerationand under -10 precipitation will occur.

By means of auxiliary materials absorbed onto the surface of the solidbody, properties of the so-called electric double-layer and thus theZP-value can be controlled. The possibility of ZP-value regulation isillustrated by the example of the aqueous suspension of17α-hydroxy-21-acetoxycorticosterone, as specified hereinabove.

If ZP-value is measured in distilled water, at a temperature of 20° C.in the presence of different auxiliary materials, as described earlier,the following results are obtained:

    ______________________________________                                        mg · cm.sup.-3 auxiliary materials                                                         ZP mV    ±                                           ______________________________________                                        .0. (in pure distilled water)                                                                       -58.0    1.2                                            0.5 sodium stearate   -74.0    2.0                                            0.5 Tween-80          -81.0    1.4                                            0.5 + 0.5 sodium stearate +                                                                         -140.2   4.1                                            Tween-80                                                                      ______________________________________                                    

We claim:
 1. A method for determining the zeta-potential of a materialsuspended in a liquid medium which comprises,(a) streaming thesuspension in a laminar flow, (b) subjecting the laminarly flowingsuspension to the effect of selectively applied and controlled D.C.voltage, (c) selecting a particle for optical examination anddetermining the electrophoretic velocity from induced changes of flowvelocity in the selected particle as a function of the applied voltage,and (d) utilizing said determination to calculate the zeta-potential. 2.The method of claim 1, in which,(a) the laminarly flowing suspension issubjected to said controlled D.C. voltage until the motion of saidselected particle stops, (b) measuring the applied voltage and obtainingthe flow velocity of the particle as electrophoretic velocity, and (c)thereafter using said determination to calculate the zeta-potential. 3.Apparatus for use in determining the zeta-potential of particles of amaterial suspended in a liquid medium comprising;(a) an injector meansfor containing and delivering by laminar flow a sample of the materialto be tested; (b) a transparent measuring pipe for receiving at one endsaid sample from said injector means; (c) vessel means for receivingsaid laminarly flowing sample from the other end of said transparentpipe; (d) connecting pipe means linking said infector means and saidinjector means and said vessel means to said measuring pipe; (e) spacedelectrodes supported in said measuring pipe; (f) microscope means havingan ocular micrometer adjacent to said measuring pipe of sufficientoptical power to observe suspended particles; (g) voltage supply meansfor energizing said electrodes with D.C. voltage; (h) voltage controlmeans for adjusting and measuring the D.C. voltage applied to saidelectrodes for stopping the motion of an observed particle.